1. Field of the Invention
The present invention relates to an apparatus and method for processing X-ray images, and more particularly to an apparatus and method for generating a continuous X-ray image of a relatively uniform density.
2. Description of the Related Art
X-ray image processing apparatuses are applicable to medical and industrial uses. Generally, in an X-ray image processing apparatus, X-ray radiation is emitted from an X-ray tube passing over an examining body, e.g., a patient, and is detected by an X-ray detector provided so as to face the X-ray tube. The detector converts X-ray data that penetrates through the examining body into analog electrical signals. The resulting electrical signals are converted to digital image data through an analog/digital (A/D) converter provided in the X-ray image processing apparatus. The digital image data are used for various image processing in the X-ray image apparatus. The processed digital image data are displayed on a monitor as X-ray density images or printed on a film by a laser imager. It is possible to store the processed digital image data in a storage medium provided in the apparatus or in an external storage medium. Further it is possible to transfer digital image data from the image processing apparatus to remote equipment through networks.
Generally, an X-ray diaphragm is attached to an exit window of an X-ray tube. Thus, the X-ray tube and the X-ray diaphragm constitute an X-ray generating device. The X-ray generating device and a detector are interlocked through a linked motion frame so as to move while keeping a linked position. The linked motion frame is movably supported by rails provided on a ceiling of an inspection room or by a bed frame in order to move the interlocked devices in a wide range from the top to the end of an examining body without moving the examining body.
Such a wide rage of movement of the interlocked pair of an X-ray generating device and a detector for obtaining X-ray images at various positions is used for, as one example, in lower legs angiography as an X-ray image diagnosis. For the lower legs angiography, a wide range X-ray image is needed from the abdomen of a patient to his feet tips while following an injected contrast agent. However, at present, there is no X-ray radiating and X-ray detecting equipment that can cover such a wide range at once. Accordingly, it has been proposed to obtain X-ray images of such a wide range by dividing the whole area into a plurality of consecutive divisional areas. Thus, once after injecting a contrast agent from a patient abdomen, a plurality of divisional X-ray images are obtained while following the injected contrast agent ridden on an aorta blood flow from the abdomen to the feet tips in order to avoid extreme burden for a patient.
For performing such divisional X-ray imaging, there are two types of methods including a stepping method and the bolus chase method. The stepping method repeats intermittent operations of X-ray imaging, stopping and movement by the pair of the X-ray generating device and detector. Namely, when the linked pair of the X-ray generating device and the detector has completed an X-ray imaging operation at a position for detecting a contrast enhancing agent, the pair is quickly moved to a next imaging position at a faster speed than the flowing speed of the contrast enhancing agent and stays until the contrast enhancing agent arrives to that imaging position. The pair performs an X-ray imaging operation at an arrival time of the contrast enhancing agent to the position. By repeating such intermittent operations, a plurality of divisional X-ray imaging covers the whole of the examining area of a wide range. The bolus chase method performs divisional X-ray imaging operations by consecutively moving the linked pair of the X-ray generating device and detector while chasing the flow of the contrast enhancing agent. Thus, the X-ray imaging operation is always completed during the movement of the linked pair of X-ray generating device and detector. For lower legs angiography, a plurality of divisional X-ray images obtained by these methods are pasted so as to generate a continuous whole image of blood vessel of a wide range from a patient abdomen to his foot tips.
However, when such divisional X-ray imaging operations are repeated in such a wide range for lower legs angiography, the detected amounts of X-rays penetrated through a patient are largely varied due to influences of body thickness and bones in the patient, or due to directly incident X-rays that do not penetrate the patient body. Consequently, for the lower legs angiography to perform the divisional X-ray imaging operation, the X-ray amount needs to be changed at every position for the divisional imaging operation. When the X-ray amounts are changed at each imaging position, a large variation of pixel values appear among the plurality of divisional X-ray images. Thus, when such divisional X-ray images of varied pixel values are pasted for generating a continuous image, it lacks continuity due to the density variation among the divisional X-ray images. Thus, the deficiency of density continuity in the continuous image makes diagnosis difficult. Especially, when the bolus chase method is applied, an X-ray imaging operation needs to be performed in a very short time in order to avoid shading off of images due to the constant movement of the linked pair of the X-ray generating device and the detector. Consequently, since it becomes harder to control the amount of X-ray radiation, the deficiency of density continuity appears in the continuous image. Thus, diagnosis becomes much harder.
In order to improve a diagnostic ability by using a continuous image, such variations of image density need to be abolished among the divisional plurality of X-ray images so that the pasted continuous image can be seen as smooth as possible in the whole range. In particular, when such density variations are extremely large among divisional X-ray images, it becomes very important to make each of image displayed densities uniform by manually controlling gradations for each of the images. However, such a manual control of gradation, i.e., density, is extremely burdensome and time consuming for a reader since divisional lower legs angiography requires a lot of divisional X-ray imaging operations. Thus, the deficiency of the conventional methods worsens throughput of the inspection.